Aqueous alkali metal chlorate solutions, in particular sodium chlorate solutions, are usually produced by the electrolysis of alkali metal chloride brine in electrolytic cells. It is known to electrolyze brine to produce hydrogen, chlorine and alkali metal hydroxide and to make alkali metal hypochlorite and hypochlorous acid therefrom within the electrolytic cell. It is also know that hypochlorite and hypochlorous acid can be converted to chlorate and chloride ions according to the equation: EQU 2HClO+ClO.sup.- .fwdarw.ClO.sub.3.sup.- +2H.sup.+ +2Cl.sup.-
Thus, in summary, within the electrolytic system, alkali metal chloride is, in effect, combined with water to form alkali metal chlorate and hydrogen gas. The lectrolysis takes place, typically at 60.degree.-90.degree. C. in electrolytic cells comprising precious metal or metal oxide coated titanium anodes and steel cathodes. It is usual to add sodium or potassium dichromate to the solution contained in the cells in order to improve current efficiency. It may be noted that the species of the chromium containing ions (CrO.sub.4.sup.=, HCrO.sub.4.sup.-, Cr.sub.2 O.sub.7.sup.=) depend on the pH value and the temperature of the solution.
The alkali metal chloride brine used as feed for the cells is normally obtained by dissolution of raw salt containing various impurities, which are detrimental to the electrolysis. The impurities are mainly present in the brine as Ca, Mg and SO.sub.4 ions.
A part of calcium ion, when introduced into the electrolytic cell, forms a deposit on the cathodes. This increases the electrical resistance of the cell and results in higher operating costs due to the consumption of additional electric energy. It is the normal practise to treat the brine before introduction to the electrolysis cells with sodium carbonate and sodium hydroxide to reduce both the calcium content of the feed brine and its concentration of magnesium.
Although the effects of calcium may be reduced by primary treatment of the brine with chemicals, there remains some calcium in the brine which accumulates within the cell, resulting in an increase in electrolytic power consumption and, thus, an increase in operating costs. In recent years, it has become more common to add, after the chemical treatment of the brine, a secondary purification using ion exchange resins developed for the removal of calcium and magnesium from brine solutions. These resins remove calcium and magnesium to levels of less than 50 ppb, typically 25 ppb. This secondary purification process is particularly advantageous in areas of high electric power costs.
Sulfate ion disrupts the electrolysis only if its concentration reaches a certain level. The electrolysis may be carried out to produce chlorate as a liquor, but more and more, said chlorate is produced as a crystal. By suitable selection of the crystallization process conditions, the chloride may be kept in solution so that, after subsequent separation of the essentially pure crystal chlorate from the mother liquor, said mother liquor may be recycled to the electrolytic cells. Said recycling causes a continuous increase of the sulfate level: the sulfate of the raw material thus enters the electrolytic system and remains in the mother liquor after crystallization and is thus recycled to the cells. At sufficiently high sulfate concentration, sulfate adversely effects electrolytic power consumption and causes operating problems due to localized precipitation in the electrolytic cells. Consequently, it is compulsory to limit the sulfate concentration to an acceptable level in the electrolysis loop.
Several methods may be considered to control said sulfate concentration in crystal chlorate plants, each with its attendant disadvantages.
It is possible to maintain sulfate in the system at an acceptable concentration by means of a liquor purge, that is, an export of chlorate solution. However, the minimum proportion of total production which must be exported as liquor is then fixed by the sulfate in the salt, not the market demand, which proportion can be large, depending on the sulfate concentration in the incoming salt or brine. Furthermore, this liquor product takes with it sodium or potassium dichromate, which represents an expense to replace, and a cost to remove if it is not acceptable in the liquid product. This method of operation requires a secure outlet for the sale of the liquor, which is of reduced economic value due to higher shipping costs. It also sets the upper limit on the proportion of the plant output which may be shipped as crystal.
It is also possible to precipitate sulfate together with chromate by cooling to a low temperature a derivated stream of mother liquor at the outlet of the main chlorate crystallizer. Such a process is described in U.S. Pat. No. 4,702,805. A part of the mother liquor at the outlet of the NaClO.sub.3 crystallizer (working under vacuum at 40.degree. C.) is cooled at -5.degree. C. The main disadvantage of such a process is the energy consumption to lower the temperature of the mother liquor from 40.degree. C. to -5.degree. C. In order to reduce said energy consumption, the electrolysis is run with a relatively high sulphate concentration which can damage the anode coating on one side and lower the cells efficiency by high oxygen production on another side.
An alternative method for controlling sulfate concentration is the reaction of the feed liquor to the crystallizer or mother liquor from the crystallizer, in whole or in part, with chemicals wich form sulfate compounds that are relatively insoluble in the liquor. Typical examples are the reactions with barium chloride or barium carbonate, in order to form barium sulfate, and the reaction with calcium chloride to form calcium sulfate. In some cases, the reaction with barium compounds is preferred, particularly, in those plants employing ion exchange treatment of the brine to prevent the introduction of calcium to the electrolytic cells. However, the process has several disadvantages.
A major disadvantage is that the addition of excessive quantities of barium compounds will result in excess barium entering the electrolytic cells. This barium forms sulfate deposit on the anode coating that is deleterious to cell operation. In addition to the reaction with sulfate ion, the barium will also combine with chromate to form barium chromate, and thus, sufficient barium must be added to react with chromate as well as sulfate. Part of the value of the barium added is therefore lost. Barium compounds and sodium or potassium dichromate are expensive, and this represents a significant waste of chemical reagents. The resulting barium sulfate and barium chromate sludge must be separated, and the resulting solids disposed of. This represents significant capital and operating costs.
Yet another disadvantage is that the solids produced by either the barium or calcium treatment will be contaminated with chromium in the form of chromate or dichromate which is considered environmentally undesirable.
However, there has been described in U.S. Pat. No. 4,636,376 a method to remove sulfate from alkali metal chlorate solutions, said method involving a chemical reaction with a calcium-containing material such as calcium chloride and producing a precipitate in which the chromium level is reduced (about 100 ppm). This result is achieved by such operating conditions that sulfate ions are precipitated as glauberite, a double salt of formula: Na.sub.2 Ca (SO.sub.4).sub.2.